1. Field of the Invention
This invention relates to illuminators capable of illuminating an object plane with a spot of variable size and more particularly to variable spot size illuminators providing a constant convergence angle suitable for use with laser photocoagulators.
2. Description of the Prior Art
Certain surgical procedures on the retina of the eye are performed using a laser photocoagulator combined with a conventional ophthalmological slit lamp. Microscopic lesions are produced in the retina and related tissues by the heating effect of a sharply focused beam of a laser, for example to reattach a detached retina. In these procedures the ophthalmologist observes the fundus of the eye through the slit lamp microscope while directing the laser beam onto the desired portion of the retina by means of a movable mirror. The spot must be small both to keep the area of surgical injury small and to assure that the energy density of the beam is great enough when absorbed by the relatively opaque retina to raise the temperature of the microscopic spot and cause a local coagulation of the tissue. However, to reach the retina the beam must first pass through the anterior portion of the eye, particularly the cornea and crystalline lens, where concentrated absorption of energy is not desired. These structures are relatively transparent, in contrast to the relatively opaque cornea, and therefore do not absorb much energy from the beam. However, for greatest safety it is desirable to have the cross section of the beam in that region as large as possible to keep the energy density low. Under these conditions, any absorbed energy is diffuse and does not cause local heating with injury to the delicate anterior structures of the eye. In order to attain the goal of a high energy density at the retina and a low energy density at the anterior portion of the eye, a convergent beam is formed by an objective lens. The convergent beam has a large cross section when passing through the anterior portions of the eye, but is focused to a high-intensity spot at the retina. The convergent beam therefore minimizes the possibility of injury to the cornea, crystalline lens, and the like.
It is also desirable in laser photocoagulation surgery to vary the focused spot size of the laser beam to produce a larger or smaller area of coagulation as required by a particular surgical procedure. In currently available laser photocoagulators the focus of the objective lens must be adjusted as the spot size is changed. However, in so adjusting the focus of the lens, the convergence angle of the laser beam is necessarily changed. This means that for some settings of the instrument the beam passing through the anterior portions of the eye has a smaller diameter than necessary and a correspondingly greater energy density, which increases the possibility of damage to the anterior portions of the eye. Accordingly, it is desirable to keep the convergence angle of the laser beam at the largest possible value consistent with passing through the anterior portion of the eye while still permitting the spot size to be adjusted.
A further disadvantage of the variable focus mechanism used to adjust spot size in the currently available laser photocoagulators is its relatively complex system of lenses within the instrument.
Variable spot size illuminators are known wherein an illuminated variable aperture is imaged on a plane to be illuminated. Such an instrument adapted to a surgeon's headlamp is disclosed by Kloots, U.S. Pat. No. 4,104,709, who discloses an instrument wherein light is supplied to the headlamp via a fiberoptic cable. The light emerging from the end of the cable is condensed by a condenser lens system and focused onto the objective lens. Immediately after the condensing lens system is located a variable aperture diaphragm of the adjustable iris type which is imaged by the objective lens into the plane to be illuminated. Evidently, while this system is suitable for its intended purpose, the use of a variable aperture diaphragm in this configuration necessarily diminishes the intensity of the illumination when the aperture is reduced. This is undesirable in a laser photocoagulator wherein maximum intensity may be required even with a small spot size.
Another illumination device using a fiberoptic light transmission means is disclosed by Weyrauch, U.S. Pat. No. 3,600,568. In this device, light from a source is imaged by a lens and an image of that lens is projected onto the incidence surface of a fiberoptic light conductor. Light emerging from the fiberoptic light conductor is imaged into the plane to be illuminated. However, Weyrauch does not disclose any means for providing an illuminated spot of variable size in the image plane.
Therefore a need has continued to exist for a variable spot size illuminator suitable for use with a laser photocoagulator which provides convenient variation of the spot size, and constant convergence angle without limitating beam power.